1. Field of the Invention
The present invention relates to a method of mounting a plurality of electronic parts on a circuit board, and also to a method of producing adhesive-coated electronic parts suited for use in the mounting method.
2. Description of the Related Art
With a recent trend to smaller-sized, thinner electronic parts such as semiconductor chips ("electronic parts" referred to herein include resistors, capacitors, semiconductor chips, etc. mounted on circuit boards), circuits and electrodes used in such electronic parts have increased density and finer connection pitch. Since fine electrodes are difficult to connect by soldering, recently, connection methods using adhesive are widely used. The connection methods include a method in which electrically conducting particles are mixed in an adhesive, and contact bonding is performed to achieve electrical connection in the thickness direction of the adhesive (e.g., Unexamined Japanese Patent Publication (KOKAI) No. 55-104007). There is another method in which no conducting particles are contained in an adhesive, and contact bonding is performed to achieve electrical connection through direct contact of fine irregularities on the electrode surfaces (e.g., Unexamined Japanese Patent Publication (KOKAI) No. 60-262430).
The connection methods using adhesive permit connection at relatively low temperatures and also provide excellent reliability because the interconnecting portion has flexibility. In addition, in the case where adhesive formed into a film or tape is used, it is possible to supply the adhesive of uniform thickness in the form of a long strip, whereby the mounting line can be automated. Also, a simple step of applying heat and pressure simultaneously attains electrical connection between electrodes of the semiconductor chip and the circuit board and mechanical connection of the two through bonding. This is why the connection methods using adhesive are attracting attention.
In recent years, multi-chip modules (MCM) which employ a, more elaborate form of the above methods and in which a large number of chips are mounted at high density on circuit boards of relatively small size are drawing attention. In general, an MCM is fabricated by forming an adhesive layer on a circuit board, peeling a separator, if any, from the adhesive layer, and positioning chips such that their electrodes face corresponding electrodes on the circuit board, followed by bonding of the electrodes. Forming an adhesive layer on a chip instead presents the problem that a complicated apparatus is required because a chip having a smaller area than the circuit board needs to be applied with an adhesive layer.
Electronic parts used in MCM include a variety of chips such as semiconductor chips, active elements, passive elements, resistors and capacitors.
Thus, various types of chips having different sizes (areas, heights) are mounted on MCM. When connecting chips to a circuit board, however, a problem arises which is not associated with conventional techniques such as the method of forming an adhesive layer on a circuit board or the heat-pressure bonding method.
Specifically, in the case where the adhesive used is in the form of a film, adhesive strips (adhesive tapes) with different widths are needed depending on different chip sizes. In MCM, however, multiple chips are mounted at high density on a small-sized circuit board, and thus only a small mounting space is available, making it difficult to use a variety of tape widths. Also, use of various tape widths increases the labor involved in the management of materials. Further, since different devices for feeding, contact bonding, tape rewinding, etc. are needed for individual tape widths, the mounting apparatus is inevitably increased in overall size and is complicated, requiring a large installation space and increasing the cost.
An attempt has therefore been made to mount various sizes of chips after an adhesive layer is formed on the entire surface of a circuit board (Examined Japanese Patent Publication (KOKOKU) No. 61-27902). With this method, however, much labor is required to remove the remaining adhesive from non-connecting sections, and also the cost increases because the adhesive layer is formed uselessly on regions other than the mounting sections. Further, since the adhesive is applied to the entire surface of the circuit board, heat applied at the time of connection can adversely affect adjacent chip mounting sections. For example, the reaction of the thermosetting adhesive may progress to such an extent that the adhesive on an adjacent section where a chip is not yet mounted becomes unusable, or an adjacent chip may develop a connection defect as the adhesive softens due to the connection heat even after the chip is mounted. This is also the case with removal of a defective chip after chip mounting. Namely, it is difficult to peel off a defective chip and also to remove the adhesive because of the reaction of the thermosetting adhesive.
Also, as an attempt to form an adhesive layer with a size substantially equal to the chip size, Examined Japanese Patent Publication (KOKOKU) No. 4-30742, for example, discloses forming an adhesive layer on a wafer and then subjecting the wafer to full dicing. In this case also, various types of adhesive-coated wafers must be prepared for different types of chips, making the process control complicated in view of the shelf stability life of the adhesive.
Unexamined Japanese Patent Publications (KOKAI) No. 63-276237 and No. 2-199847, for example, disclose applying an adhesive only to the top faces of bump electrodes (also merely called bumps) on a chip, in order to reduce the connectable pitch. However, since the adhesive is applied only to the top faces of the bump electrodes, the bump electrodes are bonded to a circuit board only in areas around the bump electrodes, so that the bonding strength and the connection reliability are low. In order to also apply the adhesive to regions other than the top faces of the bump electrodes, an underfill material needs to be poured, which, however, complicates the process and increases the cost.
Further, in the case where chips with different heights are mounted or chips are mounted on both surfaces of a circuit board, heat and pressure cannot be uniformly applied by using conventional techniques generally employed, such as a press method in which a chip-carrying board is clamped by parallel mold elements or a pressure roll method using parallel rolls. Thus, it is impossible to connect fine electrodes in this situation.